Apple itself has implemented some AFP-related restrictions, which have also been mentioned in English discussion forums.
https://www.reddit.com/r/MacOS/comments/1negm9g/nas_for_time_machine_backups_apple_says_no_more/
The solution mentioned on Reddit is to first disable AFP and then try using SMB3…
However, I personally haven’t updated my work macOS yet, and in my work environment, I’m not eager to update and test right away…